We're taking our two 19 yr old nephews on a cruise Monday and yes, you do need the same consent to travel and to authorize any needed medical treatments. Applies to anyone under 21.
Oh, and they have to be notarized
I should think that if they are considered a legal consenting adult in their own country who can drink (in most locations), vote, drive, and do anything else an adult can do, why would they need a letter stating they have their parents permission.....The cruise lines put the 21 age in place to prevent wild partying and drunken disorderly from the 18-21 year olds (who are sowing their wild oats until they realize that drinking is just a waste of money--money that can be spent on something else more important!
I would be offended as a adult and as a parent that a letter saying they are allowed to go would be required. After the age of 18 the parent really has no legal say in what their child does, they are allowed to make their own decisions and are responsible for their actions!
The cruise lines might do it to cover their butts...
2007 Victory & Triumph
2008 Conquest & Liberty
2009 Valor, Freedom, and Splendor
2010 Glory and Liberty